Primary finding
Probable cause
The engine failure due to several loose, damaged cylinder stud nuts and hold-down bolts. Contributing to the accident was maintenance personnel’s failure to detect that the cylinder nuts and hold-down bolts were not properly secured during the engine’s most recent inspection.
Investigator assessment
Analysis narrative
The commercial pilot was conducting an agricultural application flight. The pilot reported that, while making a spray run, he heard a loud "bang" and saw a puff of smoke emanate from the upper right side of the engine, followed by the propeller seizing. He made a forced landing in a field, and the airplane collided with some bushes and then nosed over. Examination of the engine revealed several loose cylinder stud nuts and hold-down bolts that appeared to have been pushed up on the cylinder skirt. Several of the studs, particularly the No. 4 cylinder stud, were stretched and looked to be nearly pushed out of the engine crankcase. One cylinder was almost separated from the crankcase. Examination of the airplane's maintenance logbooks revealed that the engine had received a major overhaul more than 8 years before the accident. The records indicated that, since that time, inspections of the cylinder heads and the cylinder hold-down studs had been conducted in compliance with two Federal Aviation Administration airworthiness directives. The engine was last inspected about 3 months before the accident. During this inspection, maintenance personnel checked the cylinder bases and heads for cracks and loose studs; however, they did not detect that the cylinder nuts and hold-down bolts were not properly secured.
Source record
Factual narrative
On June 31, 2015, about 1130 central daylight time, a Grumman G-164A, N963X, impacted terrain after the engine lost power near Emmett, Kansas. The pilot, the sole occupant on board, received minor injuries. The airplane was substantially damaged. The airplane was registered to and operated by Precision Aerial Ag LLC of Seneca, Kansas, under the provisions of 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed at the time of the accident, and no flight plan had been filed. The local flight originated from Wamego, Kansas approximately 1100.According to the pilot's accident report, he was making a spray run when he heard a loud "bang," saw a puff of smoke emit from the upper right-hand side of the engine, and then the propeller seized. He made a forced landing in a brome field, collided with some bushes, and nosed over. Examination of the engine on-site by inspectors from the Federal Aviation Administration revealed several loose cylinder stud nuts and hold-down bolts that appeared to have been pushed up on the cylinder skirt. Several of the studs – number 4 cylinder in particular -- were stretched and looked to be nearly pushed out of the engine crankcase. One cylinder was about to separate from crankcase. Examination of the engine maintenance logbook revealed the engine had received a major overhaul on February 13, 2007. On March 10, 2010, Airworthiness Directive (A.D.) 99-11-02 was complied with by inspecting the cylinder heads. On March 2, 2011, February 22, 2012, May 30, 2013, and May 13, 2014, A.D. 56-06-02 was complied with by inspecting the cylinder hold-down studs. On July 15, 2014, the engine and propeller were removed due to a blower bearing failure. On August 10, 2014, after the engine and propeller were reinstalled, and again on March 18, 2015, the cylinder bases and heads were checked for cracks and loose studs. At the time of the last inspection, the tachometer read 6,730 hours, and the engine had accrued 506.68 hours since major overhaul. The accident site was located at 14624 A4 Road, Emmett, Kansas.